Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 142346
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
646 PM CDT Sat Sep 14 2019

.SHORT TERM...
300 PM CDT

Through Sunday...

Quite the spectacular late-summer day out there with temperatures
running in the mid and upper 70s with dewpoints that have managed to
mix out into the upper 50s with a light southwesterly breeze. Hard
to believe that a persistent (and at times robust) MCS is lurking
across western Iowa. This system has been handled atrociously by
high-resolution guidance today, and this likely stems from the
fact that persistent warm advection has been occurring into a
region of conditionally unstable air (mid-level lapse rates
approaching 8 to 8.5 C/km), but models have evidently analyzed
mid-levels that are just a touch too dry for them to realize this
instability. Latest analyses over our region show mid-levels
moistening, but it looks like our area sits just far enough away
from the eastern periphery of the steeper mid-level lapse rate
plume to greatly curtail the chances for convection. Have beefed
up cloud cover a bit across the northwest half of the CWA,
however.

The main focus on the short term portion of this forecast revolves
around the potential for a nocturnal round of showers and
thunderstorms and an attendant risk for a few strong to possibly
severe thunderstorms, as well as a flash flood potential.

Two shortwaves will approach the region tonight--a more vigorous
wave currently dropping into far northwestern Minnesota, and another
more low amplitude/sheared wave drifting out of Nebraska and
northern Iowa. As this occurs, a robust low-level jet will re-
develop at the eastern flanks of these vort maxes and under the exit
region of an 80 to 90 knot jet streak. Stout warm
advection/isentropic upglide is expected to develop later this
evening and into the overnight hours across eastern Iowa before
nosing into our CWA. Expect that elevated showers and storms will
begin to develop before midnight to our west and then expand in
coverage through the overnight into southern Wisconsin before
activity consolidates and drops into northern Illinois. There may be
some initial proclivity for the mass of convection to make good
southward/forward progress given a reservoir of drier air present
in the lowest five thousand feet, but expect that as things
saturate and the LLJ intensifies, that additional convection
should materialize atop the cold pool mainly near and north of the
I-88 corridor.

The main low-level theta-e axis does look to remain ever so slightly
displaced to our northwest during the diurnal low-level jet maximum
(late tonight and into early Sunday morning), but this should be
close enough to provide more-than-sufficient moisture transport
into the region. While individual storm motions will actually be
pretty quick towards the east-southeast (around 30-35 mph given
the expected cloud-bearing flow), shear vectors bend sharply back
around with height, which indicates a potential for activity to
build more west-northwestward into the lower-level flow,
resulting in a net motions under 10 mph or so. Still a good deal
of uncertainty exactly where the axis of heaviest rainfall will
set up, and a lot depends on where the location of the LLJ and
theta-e axis set up shop. However, the environment will favor
efficient hybrid warm rain processes with PWATs approaching record
territory for mid-September and trends in today`s multi-model
consensus and HREF guidance point to an increasing heavy rainfall
threat. As a result, felt it prudent to hoist a Flash Flood Watch
for our I-88 and north locales where multi-inch rain totals have
been observed over the last several days. The main window of
concern is during the 3-10 AM timeframe.

Finally, there is also a lower-end threat for some stronger to near-
severe thunderstorms with this activity. While instability doesn`t
look all that impressive, pockets of steep mid-level lapse rates,
combined with strongly veering flow all the way to the equilibrium
levels portends a hail threat with large hail parameter values
inching past 5 to 6 in spots. We earlier coordinated a low severe
risk with the SPC for our northern tier of counties as a result.

Activity should incrementally diminish through the evening, although
think this process may be a bit slower than some guidance advertise
given the copious upstream moisture and still robust low-level warm
advection. Hi-res guidance is hitting the potential for gusty
southerly winds due to a wake low/momentum mixing combo pretty hard
through Sunday morning, so we`ll need to keep an eye on this
potential. We`ll probably have a hard time mixing out the low cloud
cover as a subsidence inversion settles and builds overhead, and
have knocked high temperatures down a bit as a result. Forecast
soundings look capped Sunday afternoon, but there will be a
considerable reservoir of CAPE available as a cold front slips into
the area. Have limited precip mentions to slight chance and showers
at this point given the degree of capping and limited overall
forcing for ascent.

Carlaw

&&

.LONG TERM...
300 PM CDT

Sunday Night through Saturday...

Concerns at the start of the period include a small chance for
showers and maybe a few thunderstorms during the evening, followed
by potential for fog overnight into Monday morning. Thereafter,
much of the week looks to be primarily quiet and unseasonably
warm, with some small precip chances later in the week.

With a warm, moist and unstable (2000+ J/kg MLCAPE) in place
across the area early Sunday evening as a weak cold frontal zone
drops south into the area, have slight chance range PoPs during
the evening, stair-stepping south a bit into late evening. Large-
scale forcing will be lacking, so will need to rely on low-level
frontal convergence at an unfavorable diurnal time this late in
season to get any showers/storms to go. In addition to lacking
large-scale forcing, models indicate a much weaker mid-level
lapse rate environment in the wake of the morning convection, also
lowering confidence on how robust updrafts may become. Thus,
while a few showers/storms are possible, it`s likely most areas
remain dry during the evening.

Attention overnight into Monday morning turns to potential for fog
development as the weak front slips south with little/no low-level
dry advection and light northerly winds. Main uncertainty with
fog is lingering cloud cover and/or new stratus development behind
the cold front. Have areas of fog mentioned along/northwest of
I-55 and outside of Chicago and patchy fog for the rest of the
area. MOS guidance suggests that dense fog will certainly be
possible if conditions become very favorable for radiational fog
formation.

The mid-upper level and surface patterns will be pretty stagnant
through the first half of the work week. Surface high pressure
ridging over the Great Lakes will extend its influence over our
area. Aloft, troughing in the West will set up unseasonably
amplified summer-like mid-upper heights and summer-like low-mid
level thermal profiles over the region. After any fog erodes
Monday morning, thermal progs support well into 80s area wide,
though will need to watch for scenario depicted by NAM/WRF
guidance which is much more aggressive with post frontal stratus
through Monday. This scenario would keep at least northern
portions of CWA cooler. Either way, onshore winds will certainly
keep lakeshore areas cooler than inland.

On Monday night, if skies clear out, low temps in outlying
locations are likely to fall below Monday afternoon dew points, so
it again could be a night to monitor for fog development, some of
which could be dense. Both Tuesday and Wednesday appear similar
with summer-like 80s inland, though humidity should stay in check,
as dew points may mix out a bit with heating/mixing. Lakeside
areas will be a bit cooler in the 70s.

Uncertainty with the large scale pattern increases a bit 2nd half
of the week as GFS indicates another front crossing the area
Wednesday night-Thursday while ECMWF keeps pattern more stagnant
and mostly capped with nocturnal MCSs possible but primarily west
of the local area. Cold front that would finally sweep out summer-
like air mass may be delayed until after day 7 as we approach the
autumn equinox. Given the overall uncertainty, felt comfortable
maintaining periodic low precip (shower/thunder) chances Wednesday
night through Saturday.

Castro

&&

.AVIATION...
For the 00Z TAFs...

The forecast concerns for the Chicago area TAFs through Sunday:

- Thunderstorms overnight and very early Sunday morning, with some
  chance of lingering at least nearby into mid-morning

- Potential for a brief period of low-level wind shear overnight

- Gusty southwest winds likely Sunday morning and early afternoon,
  that may be more south for a period in the morning

- Potential for MVFR clouds Sunday morning

- Chance for showers Sunday evening

An area of thunderstorms in central Iowa early this evening is a
sign of the returning moist air aloft, and there should continue
to be a festering of this activity for a few hours before a
blossoming into northwest Illinois and eventually north central
Illinois by 06Z. A continued growth of these storms is very
likely across northern Illinois during the overnight, with some
organization favored to result in propagation southeastward by
daybreak. This is a little quicker timing than earlier thought
and have scooted up the storm timing onset a bit. As for the end
time of storms, this becomes trickier as moist air and ascent
will continue over the area through the entire morning, likely
resulting in some regeneration over parts of northern Illinois.
At this time, this looks most favored southwest/south of Chicago
by 14Z-15Z, but may be close.

There is a signal of a wake low during the immediate time after
the storms, which would result in gusty winds probably from the
south more so than southwest. Confidence on this remains low-
medium, but winds will still increase during the morning and the
question is just how quickly. Expect some gusts into the mid 20
kts and if a wake low there could be a period of even 35+ kts.

With the moisture around in the wake of the rain, MVFR clouds are
certainly possible for the rest of the morning and even early
afternoon. Winds should eventually turn more west southwest and
lighten during mid afternoon, before a shift to northwest with a
cold front passage early evening. This front has enough moisture
and focus to have scattered showers with it, with the potential
for thunder Sunday evening being mainly south of Chicago.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014...1 AM Sunday to 10 AM Sunday.

IN...None.
LM...None.
&&

$$

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